When we think about investments we imagine investors with big sums of money, so for btc casino it would be 1 or more btc, for instance. However, the minimum amount you can invest is usually 0.01btc
Most of the casinos are now starting to use mBTC to make their users place bets with less than a 0.01 btc.
That is only because bitcoin has increased a lot during the last few months, from $1000 to more than $15,000, and we can not gamble with $150 anymore for a single bet, specially on those games in where it has a simple interface.
But you can not say that gambling is an investment, that is a huge mistake and you really need to improve that .